1. Field of the Invention
The present invention relates to a method for annealing thin film superconductors made of an oxide having a layered structure including at least Cu--O layer.
2. Related Art
There have been known niobium nitride NbN and/or germanium niobium Nb.sub.3 Ge being binary compounds of A 15 type as superconducting materials but their transition temperatures Tc are 24K at the most. Contrary to this, perovskite trivalent compounds are expected to have a higher transition temperature Tc and, in fact, there has been proposed a high temperature superconductor of Ba--La--Cu--O system (J. G. Bendorz and K. A. Muller, Zeitschrift fur physik B-Condensed Matter, 64, 189-193 (1986)).
Further, some of oxide superconductors are discovered to have critical temperatures higher than the temperature of liquid nitrogen (77.3K) and, therefore, contribute to widen the field of application of the superconductor. Especially, a Bi--Sr--Ca--Cu--O superconductor formed by a Bi complex oxide having a layered structure is found to have a transition temperature higher than 100K (H. Maeda, Y. Tanaka, M. Fukutomi and T. Asano, Japanese Journal of Applied Physics, 27, L209-210 (1988)).
Recently, it is reported that a Tl--Ba--Ca--Cu--O material makes a transition to a superconducting state at a temperature higher than 120K and is a complex compound having a layered structure similar to the Bi-superconductor (Z. Z. Sheng and A.M. Hermann, Letter to Nature, 332, 138-139 (1988)).
The mechanism for causing the superconductivity in materials of this type is not made clear in detail but there is a possibility that the transition temperature is made higher than a room temperature. Accordingly, the trivalent complex compounds of this type are expected to have much more excellent properties than those of the binary compounds mentioned above.
In order to obtain high temperature thin film superconductors having excellent superconducting properties, it is necessary to improve the crystal state of the thin film. According to the conventional method, the substrate on which a thin film is formed is heated up to a high temperature to obtain a regular crystal state. However, the annealing method by heating the substrate has a disadvantage in that crystal grains are grown thereby. These crystal grains make the surface of the thin film uneven and, thereby, it becomes impossible to make a super-fine work of an order of sub-micron on the thin film in order to form various superconducting devices.
In order to obtain even thin films, there is proposed a method in which a thin film is formed by growing a monocrystalline layer on a monocrystalline substrate kept at a low temperature. In this method, SrTiO.sub.3 and MgO is used as a substrate. Further, LaAlO.sub.3 and LaGaO.sub.3 having small dielectric constants are noticed as substrates taking into consideration of future use of superconducting devices in super high frequency field. However, these substrates are very expensive and hard to obtain. Further, the film formation conditions for growing the monocrystal homogeneously on the substrate are restricted by the structure of the forming apparatus and it is difficult to obtain superconducting thin films of large areas with a good reproducibility.
Especially, Tl--Ba--Ca--Cu--O or Bi--Sr--Ca--Cu--O materials exhibiting the highest critical temperature at the present needs a high temperature process higher than 850.degree. C. which causes the growth of grains. No method for forming thin films with a low temperature process is realized by the present and, therefore, no superconducting thin films having smooth flat surfaces have been obtained.